In the fabrication of multilayer composite structures in integrated circuits, it is often necessary to use isolation or barrier layers to minimize reactions of elements in two adjacent layers. To this end, multilayer structures sometimes include an isolation layer of a material selected because it does not react with the two layers it is separating.
An example of this would be forming a multilayer composite structure where a metal silicide may be used over a semiconductor substrate. An example of this multilayer composite structure would be in the fabrication of a pass transistor in an MOS dynamic random access memory. A first layer may be, for example, a source-drain region fabricated in or on a doped semiconductor substrate, and a second layer may be an electrical interconnect layer made of a metal silicide which facilitates electrical connection between the source-drain region and a metal conductor layer.
It has been found that the metal component (and, to a lesser extent, the silicon component) of the metal silicide tends to attract the dopant away from the interface region between the metal silicide and the substrate thus leaving the area of the substrate adjacent to the interface dopant depleted. The dopant tends to form compounds with the metal component of the silicide along the interface and/or at the grain boundaries in the metal silicide adjacent to the interface. This depleted region in the substrate can cause the performance of the semiconductor device to degrade, or possibly become dysfunctional. Furthermore, an unwanted compound layer can exhibit low electrical conductivity, which can also have these deleterious effects. The article by Murarka and Williams, "Dopant Redistribution In Silicide-Silicon And Silicide Polycrystalline Silicon Bilayered Structures", J. Vac. Sci. Technol. B5(6), Nov./Dec., 1987, pp. 1674-1688 (and references cited therein) describes this phenomenon.
Another article by C. B. Cooper and R. A. Powell, "The Use Of Rapid Thermal Processing To Control Dopant Redistribution During Formation Of Tantalum And Molybdenum Silicide/N+ Polysilicon Bilayers", IEEE Electronic Device Letters, Vol. EDL-6, May, 1985, p. 234, describes one method of dealing with dopant redistribution. There it is proposed to control dopant redistribution by rapid thermal processing. While this method can minimize the diffusion of dopants, it cannot eliminate the redistribution of dopants over a distance range of 0.001 to 0.01 microns adjacent to the metal silicide-semiconductor interface.
U.S. Pat. No. 4,784,973 to Stevens et al. for Semiconductor Contact Silicide/Nitride Process with Control for Silicide Thickness discloses a process for forming a control layer between contact openings and transistor terminal regions in an integrated circuit. According to a first aspect of the invention, a thin control layer preferably comprising a compound of silicon, oxygen and nitrogen is formed by a thermal treatment. A layer of transition metal is then deposited over the contact region, and reacted during another thermal treatment to form a compound of, for example, titanium, silicon, oxygen and nitrogen. The control layer is believed to retard the rate of diffusion of silicon into the titanium, thus preventing undesirable "spikes" and resultant transistor failure.
A second aspect of the invention disclosed in U.S. Pat. No. 4,784,973 includes depositing a layer of a transition metal, such as titanium, over the above-described structure, and thermally reacting this structure in a nitridating environment, thus substantially converting this layer into titanium nitride. The layer thus formed acts as a barrier to silicon diffusion during subsequent thermal cycles.
While the reaction barrier formed according to U.S. Pat. No. 4,784,973 is effective in retarding silicon movement, this barrier has only limited effectiveness in retarding the movement of dopants, such as boron, from silicon into adjacent metal silicides. Furthermore, formation of a reaction barrier according to the disclosure of U.S. Pat. No. 4,784,973 requires the addition of special depositions and thermal cycles to the typical sequence of semiconductor processing. Thus, this reaction barrier does not provide an entirely satisfactory means for retarding unwanted movement of dopants.
Therefore, it is an object of this invention to provide a reaction barrier which minimizes the problems caused by unwanted dopant movements in semiconductor devices.
It is a further object of this invention to provide a reaction barrier that does not require the deposition of an additional layer of material.